Life-history Trade-offs

Life History Trade-offs
LH Trade-offs
• Trade-offs have a central role in life history
theory
• Trade-offs have been experimentally
manipulated in both the lab and field,
measured as phenotypic correlations, or
genetic correlations
Life-history Trade-offs
• Trade-offs are the linkages between traits that
constrain the simultaneous evolution of two or
more traits
• Some of the most commonly studied trade-offs
include:
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–
–
–
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survival vs. reproduction
Current vs. future reproduction
Reproduction and growth
Reproduction and condition
Quality and quantity of offspring
LH Trade-offs
• One of the reliable methods for quantifying
trade-offs include measuring a particular trait
and measure the correlated responses in the
other trait
• Another approach is to manipulate the
phenotype and study the consequences in the
same individuals (e.g. clutch sizes)
LH Trade-offs
• Examples: red deer
• Adult mortality is higher in females that are
nursing offspring
• There is both a physiological and ecological
mechanism
LH Trade-offs
• What is the mechanism?
• Lactating females do not have large fat
reserves, thus causing higher over-winter
mortality
• This is also age-dependent
LH Trade-offs
• E.g. Beech Trees
• Beech trees will have ‘mast’ years
• During those years, it is not surprising the
growth ring may only be ½ as large as in
‘normal’ years
LH Trade-offs
• Beech tree trade-off
LH Trade-offs
• Grasshoppers will
trade ‘quantity’
relative to
‘quality’…possibly
based upon
environmental
conditions
LH Trade-offs
• E.g. Neotropical frogs
• Female frogs are not the only ones that hear
calling males! Bats
• The capture rate is positively correlated with
calling rate
• They can distinguish between species and size
and make ‘informed’ decisions
Life History Trade-offs
Life History Traits
Mangrove Warbler Yellow Warbler
Territoriality
Year-round
Seasonal
Breeding season
length
3.5 months
2.5 months
Bigamous male
percent
10
5 or less
Average clutch size 3 eggs
4.5 eggs
Average incubation
13 days
time
11 days
Average brooding
time
11 days
8.5 days
Depredation
percent
65
30
Nesting success
percent
26
55
Nesting attempts
2
?
Females double
brooding percent
5
1 or less
Cowbird parasitism
8
percent
40
Parental care
percent
44
57
Adult survivorship
65
50
LH Trade-offs
• There may be several types of trade-offs
• Physiological: allocation decisions between
two or more processes that compete directly
with one another for limited resources within
a single individual
• E.g. red deer, beech tree, grasshoppers
LH Trade-offs
• Microevolutionary: broader than
physiological trade-offs; include trade-offs in
which one trait increases fitness while linked
to a second trait that directly decreases fitness
• Microevolutionary trade-offs are defined by
the response of populations whereas
physiological trade-offs may exist without any
microevolutionary trade-off
LH Trade-offs
• Consider the grasshopper in which there is a
reaction norm for number and size of
offspring, but with no genetic variation for the
reaction norm
• When conditions are poor, they produce
fewer, larger eggs
• This is really a case of individual plasticity
(without a genetic component)
LH Trade-offs
• It is important to remember that physiological
trade-offs that are not genetically variable
may have been previously, but have become
fixed because they were the optimal
allocation
LH Trade-offs
• Macroevolutionary trade-offs are defined by
comparative analysis of variation in traits
among independent phylogenetic events
• Consider two traits that are not plastic and for
which there is no genetic variation (fixed)
• Within phylogenetic groups, the two traits are
negatively correlated
• Also, the traits are apparently adaptively
associated with habitats
LH Trade-offs
• Such patterns could only exist because
physiological and microevolutionary trade-offs
that existed in the past have left their traces in
an entire lineage even though we cannot now
measure them within species
LH Trade-offs
• By identifying the comparative pattern within
which the intraspecific trade-offs occur, we
identify conditions common to who lineages
• This gives greater generality to evolutionary
patterns and potentially mechamisms
Physiological Trade-offs
• Physiological ecology demonstrates the
lineage-specific effects that constrain
microevolutionary optimization – condition
thresholds for breeding, growth rates as a
function of body size, limits on maximum
performance, and the amount of energy that
it takes to produce a gram of offspring
Physiological Trade-offs
• While these traits are relatively constant
(conservative) within species, but vary among
lineages
Physiological Trade-offs
• Physiology is the basis
of phenotypic
correlations and is the
filter through which
genetic conditions are
expressed
Physiological Trade-offs
• Genome consists of
a part carrying
lineage-specific
effects
characteristic of a
species and a
variable part
carrying the
differences among
individuals
Physiological Trade-offs
• Physiological tradeoffs constrain adaptation:
with limited resources, an increase in energy
allocation must result in a proportional
decrease in materials and energy allocated to
another (the Principle of Allocation)
• What is left is after standard metabolic use is
sometimes referred to as a surplus
• It can be allocated to growth (u) and
reproduction (1-u)
Physiological Trade-offs
• What value of u will
maximize fitness?
• Given values of fitness for
every pair of values of
growth and reproduction,
one can plot the fitness
values on the growthreproduction plane and
draw contours through
points of equal value
Physiological Trade-offs
• There may be a single combination of growth
and reproduction that produces the highest
fitness, there would be a peak, with declining
fitness around it
• The trade-off should represent a straight line
(although the slope does not have to be 1)
• Where the trade-off intersects with the
highest value on the fitness contour, fitness is
maximized
Physiological Trade-offs
Physiological Trade-offs
• There are many caveats: see handout
Physiological Trade-offs
• The physiological models focuses on how
materials and energy are acquired, processed
and utilized
• It is based upon rates (e.g. feeding, metabolic,
growth…)
Physiological Trade-offs
• Consider the
fate of ingested
material for a
carnivorous fish
swimming and
foraging
optimally
Physiological Trade-offs
• Feeding constraints
and efficiencies
connect physiological
ecology, behavioral
ecology and life
history evolution
• Male Kestrals feed
females and young
Physiological Trade-offs
• Males with broods from 4 to 7 chicks all spent
an average of 4.75 hours per day in flight
independent of brood size (382kJ/day foraging)
• Males with larger broods hunted more
efficiently and provision equally well (63 g/day)
• When nestlings were manipulated (number or
quantity of food), males increased delivery
rates by almost 3x
Physiological Trade-offs
• The energy spent was extremely high and
sustained (up to 11 days)
• However, they still only foraged during half of
the daylight hours…what does that mean?
Physiological Trade-offs
• Foraging and reproductive success in geese:
geese pair bond before arriving on the
breeding grounds
• The quality of forage and efficiency by which
females can graze depend upon male status
• Consequently, dominant females return in the
fall with more young and females with
subordinate males get divorced more often
Physiological Trade-offs
• Reproductive effort is a key concept in LHE,
but costs are poorly understood
• Individuals of two species could devote the
same quantity of energy to reproduction at
the equivalent body sizes, but differ greatly in
the absolute amount of energy gathered or in
the time during which it was gathered
• However, the ratio would be equal, but true
investment is not
Physiological Trade-offs
• Second, even if energy budgets were identical
for two species, a comparison of clutch
weight/body wt ratios might not provide
comparable measure of effort if the species
differed in the number of clutches produced in
a single season
Physiological Trade-offs
• Individuals may also differ
in their ability to detect
predators, thus
determining the
investment in predator
detection is not equal
either
• Investments are difficult
to follow!
Physiological Trade-offs
• Do we really need to study or measure
reproductive effort? NO
• What we really need is the quantity of
reproduction and the cost of reproduction
(changes in B or D), but not reproductive
effort (physiological allocations)
Microevolutionary Trade-offs
Microevolutionary Trade-offs
Microevolutionary Trade-offs
Microevolutionary Trade-offs
Physiological Trade-offs
Life History Traits
Mangrove Warbler Yellow Warbler
Territoriality
Year-round
Seasonal
Breeding season
length
3.5 months
2.5 months
Bigamous male
percent
10
5 or less
Average clutch size 3 eggs
4.5 eggs
Average incubation
13 days
time
11 days
Average brooding
time
11 days
8.5 days
Depredation
percent
65
30
Nesting success
percent
26
55
Nesting attempts
2
?
Females double
brooding percent
5
1 or less
Cowbird parasitism
8
percent
40
Parental care
percent
44
57
Adult survivorship
65
50